1. Field of the Invention
According to a first aspect, an indexable turning insert has a polygonal basic shape and includes opposite top and bottom sides between which first clearance surfaces extend adjacent to main cutting edges. Pairs of main cutting edges approach each other in corners at angles of at least 55° and at most 80°. Between each pair of main cutting edges there is formed a secondary cutting edge to which a secondary clearance surface connects. The first and second clearance surfaces form clearance angles with respect to an imaginary reference plane disposed perpendicular to the top side of the turning insert.
According to a second aspect, a turning tool includes a turning insert as described above.
2. Description of Related Art
It is known to use turning tools and turning inserts for internal turning that provides a finished surface to pre-drilled holes or pre-formed holes having a diameter of less than 20 millimeters (mm). A related tool for internal turning type machining includes a basic body or boring bar having an insert seat with a turning insert detachably mounted in the insert seat. In order to accomplish internal turning, the insert seat of the related tool is coupled to the basic body so that a radially outermost portion of the turning insert extends beyond an envelope surface defined by the basic body. In order to facilitate chip evacuation when machining small holes, the basic body of the related tool includes a chip flute. The turning insert of the related tool is triangular with clearance surfaces that form a single clearance angle with respect to an imaginary reference plane disposed perpendicular to a top side of the turning insert.
Another related tool includes a turning insert that has a triangular basic shape, opposite top and bottom sides, and clearance surfaces extending between the top and bottom sides. Main clearance surfaces are adjacent to main cutting edges and meet each other in pairs in corners. Formed between each pair of the main cutting edges is a secondary cutting edge to which a secondary clearance surface connects. The main and secondary clearance surfaces again form a single clearance angle with respect to an imaginary reference plane disposed perpendicular to a top side of the turning insert.
These related tools suffer from a number of disadvantages, including that they cannot successfully machine exceptionally small holes, i.e., holes having diameters of 12 mm down to 6 mm. In particular, it is believed that attempting such machining with the related tools results in the clearance surface, which is positioned under the chip-removing tip, scraping against and colliding with the surface just generated. Another disadvantage of the related tools is that it is believed to be impossible to carry out face turning, since there is no chip breaker adjacent to a secondary cutting edge.